Add managed-by label to Pods created from TaskRuns

[imjasonh]

Changes

Also annotate the controllers' Deployments and created Pods with labels
denoting their purpose.

This is a Kubernetes best practice
(https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/)
and will allow operators to understand at a high level where these Pods
come from.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• [y] Includes tests (if functionality changed/added)
• [n/a] Includes docs (if user facing)
• [y] Commit messages follow commit message best practices

See the contribution guide for more details.

Release Notes

Label Pods created to run TaskRuns with `managed-by` to denote Tekton created them.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ImJasonH

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• OWNERS [ImJasonH]

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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/taskrun/resources/pod.go	89.0%	89.7%	0.8

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/taskrun/resources/pod.go	89.0%	89.7%	0.8

[ghost]

/lgtm